Weird z-axis issue when repeating a job

Hi all,

I am very new to CNC and to Workbee in particular. I have just noticed a weird issue. (maybe not so weird, you tell me).

I have run a job, which consists of two elliptical pockets going the whole depth of the material (20mm). The depth is cut in multiple passes, 2mm at a time. The cutting bit is 1/4’’ upcut flat, two flute one. The material is soft-ish wood (pine I think).

The first time the job ran, everything was fine, pockets were cut just right and the spoilboard was untouched - precisely as designed. Then I repeated the job on another piece of material - without modifying the work zero position in any way. This time around, the first pocket of the two was correct, but on another one, the bit cut more than 0.5mm into the spoilboard. At the end, the job finished ok (after all, the spoilboard is there to be spoiled), but this has me wondering about the precision of the z-axis. I believe I might have heared a weird noise during the last pass, as if the bit hit something, or something skipped in the works of the machine. But I am not sure, perhaps I am imagining things. I don’t think the spoilboard deflected or moved upwards during the cut, so I have non idea what could be wrong.

Perhaps anybody has some idea what could be happening?